1. Field of the Invention
This invention is concerned with a magnetic bubble device comprising a magnetic garnet film in which magnetic bubbles can propagate.
2. Description of the Prior Art
Magnetic bubble devices typically consist of a flat, nonmagnetic garnet substrate such as gadolinium gallium garnet, Gd.sub.3 Ga.sub.5 O.sub.12 or neodymium gallium garnet, Nd.sub.3 Ga.sub.5 O.sub.12, and a thin epitaxial layer of a magnetic garnet deposited on the substrate. Magnetic layers are typically of a thickness on the order of 10 micrometers and have their easy axis of magnetization perpendicular to the substrate. Deposition techniques such as liquid phase epitaxy and chemical vapor deposition are being used successfully for the creation of such layers.
Interest in magnetic bubble devices is due in large part to their potential application in communications and data processing equipment. Basic to such applications is the ability of the magnetic film, when magnetically biased in a direction perpendicular to the film, to sustain small domains magnetized in a direction opposite to the direction of the bias field. These domains are of cylindrical shape and extend from the surface of the magnetic film to the vicinity of the film-substrate interface. They typically are of a diameter approximately equal to the thickness of the film.
Magnetic bubbles can be created, propagated, and annihilated by appropriate local modulation of the intensity of the bias field. They retain their identity within a range of bias field strengths but disappear once the strength of the so-called collapse field is reached.
It has become apparent that special care is required to assure uniformity of magnetic bubbles. Specifically, it is essential to suppress undesirable bubbles designated as hard bubbles which are of lower mobility than normal bubbles and which move at an angle to, rather than in the direction of the magnetic field gradient. Hard bubbles are also characterized in that their collapse field strength is considerably higher than that of normal bubbles.
The problem of hard bubble suppression has received a considerable amount of attention. Among proposals for its alleviation are the multilayer deposition method described in "Multilayer Epitaxial Garnet Films for Magnetic Bubble Devices--Hard Bubble Suppression" by A. H. Bobeck, S. L. Blank, and H. J. Levinstein, published in The Bell System Technical Journal, July-August 1972, pages 1431-1435. This method consists of interposing a low-moment second magnetic layer between the substrate and the bubble sustaining layer. It was found that in the presence of this layer only normal bubbles can be sustained.
An alternate method for suppressing hard bubbles is that described in "Suppression of Hard Bubbles in Magnetic Garnet Films by Ion Implantation" by R. Wolfe and J. C. North, published in The Bell System Technical Journal, July--August 1972, pages 1436-1439, which consists of implanting hydrogen or helium ions into the free surface of the magnetic film. As a result of this implantation the easy axis of magnetization in a thin surface layer becomes parallel to the surface.
Further methods for dealing with hard bubbles have been described in "Suppression of Hard Bubbles by a Thin Permalloy Layer" by Y. S. Lin and G. E. Keefe, Applied Physics Letters, Vol. 22, No. 11, June 1, 1973, pages 603-604, "Hard-Bubble-Free Garnet Epitaxial Films: The Garnet-Permalloy Composite Structure" by M. Takahashi, H. Nishida, T. Kobayashi and Y. Sugita, Journal of the Physical Society of Japan, 34 (1973) page 1416, and in "New Approaches to Hard Bubble Suppression" by R. D. Henry, P. J. Besser, R. G. Warren and E. C. Whitcomb, presented at the Institute of Electrical and Electronics Engineers Conference on Magnetism and Magnetic Materials, 24-27 Apr., 1973, Washington, D.C.